This section provides background information related to the present disclosure which is not necessarily prior art.
FIG. 1 is a view illustrating an example of the semiconductor light emitting device proposed in U.S. Pat. No. 7,262,436. The semiconductor light emitting device includes a substrate 100, an n-type semiconductor layer 300 grown on the substrate 100, an active layer 400 grown on the n-type semiconductor layer 300, a p-type semiconductor layer 500 grown on the active layer 400, electrodes 901, 902 and 903 formed on the p-type semiconductor layer 500, while serving as reflective films, and an n-side bonding pad 800 formed on the n-type semiconductor layer 300 which has been etched and exposed. The n-type semiconductor layer 300 and the p-type semiconductor layer 500 can be of opposite conductive types. Preferably, a buffer layer (not shown) is provided between the substrate 100 and the n-type semiconductor layer 300. A chip having this structure, i.e. where all the electrodes 901, 902 and 903 and the n-side bonding pad 800 are formed on the opposite side of the substrate 100, with the electrodes 901, 902 and 903 serving as reflective films, is called a flip-chip. The electrodes 901, 902 and 903 are made up of an electrode 901 (e.g., Ag) with a high reflectance, an electrode 903 (e.g., Au) for bonding, and an electrode 902 (e.g., Ni) for preventing diffusion between materials of the electrode 901 and materials of the electrode 903. While this metal reflective film structure has a high reflectance and is advantageous for current spreading, it has a drawback that the metal absorbs light.
FIG. 2 is a view illustrating an example of the semiconductor light emitting device proposed in JP Pub. No. 2006-120913. The semiconductor light emitting device includes a substrate 100, a buffer layer grown on the substrate 100, an n-type semiconductor layer 300 grown on the buffer layer 200, an active layer 400 grown on the n-type semiconductor layer 300, a p-type semiconductor layer 500 grown on the active layer 400, a light-transmitting conductive film 600 with a current spreading function formed on the p-type semiconductor layer 500, a p-side bonding pad 700 formed on the light-transmitting conductive film 600, and an n-side bonding pad 800 formed on the n-type semiconductor layer 300 which has been etched and exposed. Further, a DBR (Distributed Bragg Reflector) 900 and a metal reflective film 904 are provided on the light-transmitting conductive film 600. While this structure reduces light absorption by the metal reflective film 904, it has a drawback that current spreading is relatively poor, compared with the use of the electrodes 901, 902 and 903.
FIG. 3 is a view illustrating an example of the semiconductor light emitting device proposed in JP Pub. No. 2009-164423. In the semiconductor light emitting device, a DBR 900 and a metal reflective film 904 are provided on a plurality of semiconductor layers 300, 400 and 500, a phosphor 1000 is provided on opposite side thereof. The metal reflective film 904 and an n-side bonding pad 800 are electrically connected with external electrodes 1100 and 1200. The external electrodes 1100 and 1200 can be lead frames for a package, or electrical patterns provided on the COB (Chip on Board) or PCB (Printed Circuit Board). The phosphor 1000 can be coated conformally, or can be mixed with an epoxy resin and then used to cover the external electrodes 1100 and 1200. The phosphor 1000 absorbs light that is generated in the active layer, and converts this light to a light of longer or shorter wavelength.
FIG. 10 is a view illustrating another example of the semiconductor light emitting device in the prior art, in which semiconductor light emitting device includes a substrate 10 (e.g., a sapphire substrate), a buffer layer 20 grown on the substrate 10, an n-type semiconductor layer 30 grown on the buffer layer 20, an active layer 40 grown on the n-type semiconductor layer 30, a p-type semiconductor layer 50 grown on the active layer 40, a current-spreading conductive film 60 formed on the p-type semiconductor layer 50, a p-side electrode 70 formed on the current-spreading conductive film 60, an n-side electrode 80 formed on an exposed portion of the n-type semiconductor layer 30 resulted from the mesa etching of the p-type semiconductor layer 50 and the active layer 40, and a protective film 90. The current-spreading conductive film 60 is provided to promote the current supply over the entire p-type semiconductor layer 50. The current-spreading conductive film 60 is formed across almost the entire face of the p-type semiconductor layer 50, and can be configured, for example, as a light-transmitting conductive film made of ITO or Ni and Au, or as a reflective conductive film made of Ag. The p-side electrode 70 and the n-side electrode 80 are metal electrodes for supplying current, which can be made of any material from the group consisting of nickel, gold, silver, chromium, titanium, platinum, palladium, rhodium, iridium, aluminum, tin, indium, tantalum, copper, cobalt, iron, ruthenium, zirconium, tungsten and molybdenum, or any combination thereof, for example. The protective film 90 is made of a material such as SiO2, and may be omitted. To meet the needs of semiconductors light emitting devices of larger areas and greater power consumption, finger electrodes and a plurality of electrodes have been adopted to facilitate the current spreading in the semiconductor light emitting device. For instance, a semiconductor light emitting device having a larger area (e.g., width/length=1000 um/1000 um) has finger electrodes for each of the p-side electrode 70 and the n-side electrode 80 in order to provide an enhanced current spreading effect, and also a plurality of p-side electrodes 70 as well as a plurality of n-side electrodes 80 such that a sufficient amount of current is supplied. However, since metallic electrodes such as the p-side electrodes 70 and the n-side electrodes 80 usually have a large thickness and there is a greater loss in the light absorption accordingly, their light extraction efficiency is deteriorated.